1. Field of the Invention
The present invention relates to a row-based Viterbi bit detection method for detecting the bit values of bits of a channel data stream stored on a record carrier. Further, the present invention relates to a corresponding row-based Viterbi bit detector, a method of reproduction of a user data stream, a corresponding reproduction device and a computer program for implementing said methods. In particular, the present invention relates to a row-based Viterbi bit detection method for information written in a two-dimensional way on a record carrier, such as an optical disc or a memory card. The present invention could also be regarded as relating to Partial Response Maximum-Likelihood (PRML) bit detection, i.e., the invention also relates to a PRLM bit detection method and device.
2. Description of the Related Art
European Patent Application No. 01203878.2 discloses a method and system for multi-dimensionally coding and/or decoding an information to/from a lattice structure representing channel bit positions of said coded information in at least two dimensions. Encoding and/or decoding is performed by using a quasi close-packed lattice structure. For the case of three-dimensional encoding and/or decoding, preferably a (quasi) hexagonally close packed (hcp) lattice structure is to be used. Another possibility in three dimensions is the use of a (quasi) face-centered cubic (fcc) lattice structure. For the case of two-dimensional encoding and/or decoding, preferably, a quasi-hexagonal lattice structure is to be used. Another possibility in two dimensions could be the use of a quasi square lattice structure. For the sake of a more simple and clear description of the object of the present invention, special attention is given to the two-dimensional case. The higher-dimensional cases can be derived as more or less straightforward extensions of the two-dimensional case. The special situation of the one-dimensional case which comprises only a single row of bits, boils down to the very classical case of PRML bit detection as is well known in the state of the art for one-dimensional modulation and coding, as, for instance, described in Chapter 7 “Viterbi Detection” by Jan Bergmans, “Digital Baseband Transmission and Recording”, Kluwer Academic Publishers, 1996.
In one-dimensional recording on optical discs, the channel bits of the channel data stream are recorded along a spiral track, the spiral being one bit wide. For two-dimensional recording, the channel bits of a channel data stream can also be recorded along a spiral, albeit a broad spiral, that consists of a number of bit rows which are aligned with respect to each other in the radial direction, that is, in the direction orthogonal to the spiral direction. The additional alignment of bit rows can also be obtained in another direction not strictly orthogonal to the spiral direction, but in a direction making a certain non-zero angle with the spiral direction.
A PRML bit detection apparatus for deriving a bit sequence from an input information signal is disclosed in WO 00/18016, corresponding to U.S. Pat. No. 6,580,766. The apparatus comprises input means for receiving the input information signal, sampling means for sampling the input information signal at sampling instants so as to obtain samples of the input information signal at said sampling instants, conversion means for converting an array of said samples into an array of bits of a first or a second binary value, detection means for repeatedly detecting a state for subsequent sequences of n subsequent bits of said array of bits, said subsequent sequences being obtained by shifting a time window of n subsequent bits each time over one bit in time, means for establishing the best path through the states, and deriving means for deriving a sequence of bits in accordance with the best path through said states. In that apparatus, n is larger than 3, and sequences of n subsequent bits having n−1 directly successive bits of the same binary value are allocated to the same state. In a specific embodiment, n is an odd number larger than 4. In that specific embodiment, sequences of n subsequent bits having n−2 directly successive bits of the same binary value as the central n−2 bits in such n-bit sequence, are allocated to the same state. This results in a PRML detection apparatus with reduced complexity.
A full-fledged PRML bit detector for 2D bit-arrays would require a trellis which is designed for the complete width of the broad spiral, with the drawback of an enormous state-complexity that leads to a completely impractical algorithm, since it cannot by far be implemented even in the fastest hardware of the coming decennia.
A 2D PRML bit detector is disclosed in “Study of Recording Methods for Advanced Optical Disks”, S. Taira, T. Hoshizawa, T. Kato, Y. Katayama, T. Nishiya, T. Maeda, Technical Report of IEICE, 2002–03, pp. 57–64. Therein, an optical storage system with 2D modulation on a square lattice, with d=1 RLL constraints both in horizontal and vertical directions is described. For this system, a receiver, consisting of a 2D-equalizer and a 2D-Viterbi detector or a 2D-PRML detector, is disclosed in “Two-Dimensional Partial-Response Equalization and Detection Method with Multi-Track”, T. Kato, S. Taira, Y. Katayama, T. Nishiya, T. Maeda, Technical Report of IEICE, 2002–03, pp. 65–70. The 2D PRML detector is based on three successive bit rows, but the typical add-compare-select operation (ACS) of the Viterbi-algorithm uses the HF-samples of the central bit row only; the other two bit rows are used in order to determine, in a joint way, the reference level from which the received HF-signal should be subtracted in order to derive the branch metrics for the branches (or transitions) in the trellis diagram of the Viterbi detector. In this way, at its output, the Viterbi-detector yields bit-decisions for the central bit row only. In this sense, for successive rows, the PRML detectors operate already independently, and the state-complexity for the complete set of bit rows has been reduced down to the complexity that is to be associated with 3 rows only. Within a strip of 3 rows, the known bit detector performs a kind of 2D-PRML, but with a 1D-output (for the single row being the mid-row of the 3-row strip). It should be noted that the channel strips are processed independently, but that the state-complexity of the Viterbi-detector is still quite high.
Assuming the practical case of a 3-taps response in the tangential direction, as disclosed in the above mentioned documents. For the square lattice, but also when applying this algorithm for a hexagonal lattice, assuming no modulation coding for both lattices, states characterized by 6 bits each would be obtained, yielding a number of 26=64 states; each state would then have 23=8 possible predecessors. On the square lattice, assuming the runlength modulation coding of the above references with 2D d=1 constraint, the number of states is only a bit smaller than 64, since some of the states are forbidden just because of the use of the runlength constraints along vertical and horizontal directions.